The controlling of servo-drives, in particular of solid-state actuators, places considerable demands on the servo-drive. For example, for charging and discharging, high voltages and high currents have to be provided for short periods of time. The servo-drive can be charged or discharged in less than a millisecond. In this time, a controller controlling the servo-drive should supply electrical energy to or remove electrical energy from the servo-drive in a controlled and reproducible manner.
Solid-state actuators are used in injection valves. Injection valves are used in internal combustion engines for injecting fuel into a combustion chamber. The solid-state actuator opens or closes the injection valve depending on a voltage applied in each case.
Document DE 198 14 594 A1 discloses a piezoelectric element in which a charging process or a discharging process is terminated a certain time before a desired voltage is attained in the piezoelectric element. As a consequence of the charge or discharge current not dropping abruptly to zero, the piezoelectric element continues to be charged or discharged for a certain time after termination of the charging or discharging.
Document DE 10 2005 016 279 A1 discloses a circuit arrangement for actuating a rechargeable and dischargeable electromechanical actuator, especially for a fuel injection valve of an internal combustion engine. The charged actuator is discharged by means of a first discharge branch which is connected to the actuator. The actuator is also discharged by means of a second discharge branch which is connected to the actuator. The second discharge branch is triggered by means of a controller to discharge the actuator after the first discharge branch while the first discharge branch is still discharging the actuator.
Document DE 10 2004 047 961 A1 discloses a device comprising a DC/DC converter, said DC/DC converter supplying at the output end a high supply voltage which is applied to a series connection encompassing a high-side switching transistor and a low-side switching transistor. A series connection comprising a high-inductance coil and a piezoactuator that is to be triggered is disposed between a junction of the two switching transistors and a reference potential. An excitation signal having a predefined pulse duty factor (effective voltage) and a predefined switching frequency is applied to the junction in order to charge or discharge the piezoactuator.
Document DE 10 2004 009 614 A1 discloses a piezoelectric actuator of an injection valve which is controlled by application of a control curve on which the maximum amplitudes of the current controlling the actuator are given. A slowly rising or falling charge course at the beginning or end of a charging or discharging phase and hence a charge level that is linearly controllable over time can thus be achieved in the actuator.
Document DE 101 14 421 A1 discloses a capacitive actuator element, in particular a piezoelectric actuator. The piezoelectric actuator is charged or discharged with a control current regulated to a constant average value until such time as a predefined voltage applies to the actuator element. This charging or discharging process results in a stroke motion through a stroke that is dependent on the control current. A variable that is dependent on the charge of the actuator element is recorded as the controlled variable. The controlled variable is preferably also dependent on the control current. In that the controlled variable is adjusted to a reference variable by controlling the control current, the time course of the controlled variable is predefined such that a predefined time course of the stroke is achieved by controlling the controlled variable.
Document DE 10 2007 014 326 A1 discloses a controllable switch which in order to charge and/or discharge a capacitive element is triggered to interrupt a charging current. The controllable switch is triggered when the charging current exceeds a predetermined charging-current threshold. The charging-current threshold is altered while the capacitive element is being charged to the predefined charging voltage. The controllable switch is triggered again in order to couple the charging current into the capacitive element if a predefined period of time after a previous triggering of the controllable switch has elapsed and/or the charging current drops below the predetermined charging-current threshold.